Optical fiber microwire devices are well known for their advantages such as strong confinement, large evanescent fields, great configurability, low-loss connection, controllable dispersion, etc. According to the nonlinear coefficient
      γ    =                            2          ⁢                                          ⁢          π                λ            ⁢                        n          2                          π          ⁡                      (                                          ω                2                            /              4                        )                                ,wherein λ is the wavelength, ω is the mode field diameter, and n2 is the nonlinear-index coefficient. Obviously, the small mode diameter leads to high nonlinearity. However, simply reducing the diameter of the fiber core of a conventional optical fiber can not achieve small enough mode area. This is owing to the fact that the confinement ability of the fiber cladding with the reduction of core diameter. Increasing the index contrast between the core and the cladding is an efficient way to confine the core modes in a small core. Optical fiber nanowires and microwires can overcome such problem. Optical fiber nanowires and microwires based couplers, high-birefringent optical fiber, and micro resonator have been demonstrated.
Optical fiber nanowires and microwires are generally fabricated by tapering, a segment of optical fiber is tapered to an outer diameter on the order of micrometer or sub micrometer or even smaller. The optical fiber nanowires and microwires is generally surrounded by air, which has low refractive index, whereas the nanowires and microwires have high refractive index. Therefore, the nanowires and microwires has high numerical aperture, as a result, even at small outer diameter, effectively guiding can be achieved. Optical fiber nanowires and microwires have some disadvantages though. For example, the transmission loss will increase with the time if no effective protection to the nanowires and microwires [Adv. Opt. Photon., 2009, 1(1): 107-161]. Therefore, nanowires and microwires should be coated. However, such process will influence the transmission characteristics of the microwires. Also the microwires are sensitive to the outer environment variations.
To overcome such disadvantages, small core optical fiber based on microstructure has been proposed. For example, by using high air-filling fraction cladding and small core size, microstructured optical fiber with high numerical aperature can be achieved, and mode diameter could be as small as 1 μm. Owing to the small core area, to effectively confine light, special fiber configurations are applied. Fore example, the Y-shaped microstructured optical fiber is used to increase the air holes surrounding the fiber core, and reducing the strips surrounding the air holes [Applied Physics B: Lasers and Optics, 2010, 98(2): 371-376]. Even so, the fiber core will still be surrounded by supporting material around the air holes, which will increase the mode area, and deteriorate the confinement ability of the air cladding.